In the first post of this series we considered the history and origins of English chalk streams; where they were and where they’d come from. In this post we’ll look at what makes them tick: their characteristics and tell-tale signs.
The technical definition of a chalk stream is any river whose base-flow index (the volume of river flow derived from groundwater aquifers) exceeds 75%, and whose course runs over chalk geology. Beyond this, chalk streams are characterised by stable planforms, low stream densities, and clear, alkaline waters. At the catchment scale, permeable rocks and soils have a high infiltration capacity, leading to dampened flood hydrographs, few tributaries and low connectivity with the landscape. At the reach scale, upstream headwaters (winterbournes) may experience a naturally dry period of low flows at the end of summer and the water table may fall because of insufficient precipitation inputs into chalk aquifers (hence the drought earlier this year).
Natural chalk rivers also have relatively low suspended sediment concentrations, high water clarity and comparatively stable thermal regimes. The shallow gradient and attenuated flood peaks of chalk rivers limit their stream power and competence, with a resultant absence of coarse gravel sedimentological features. Large chalk rivers are instead characterised by high width to depth ratios, long periods of high flows and gravel beds that experience relatively little bedload transport. Fast flows exceeding 0.1ms-1 are common, especially in 3rd order or greater streams, and these maintain a clean gravel-pebble substrate. Water temperatures are generally stable and warm, water clarity and light availability are high, and consequentially photosynthetic productivity is also high.
With few tributaries and seldom overbank flows, chalk river surface waters can be relatively poorly connected to their catchments, both laterally and longitudinally. Because sediment inputs from hillslope processes and other natural agency are minimal, any activites that increase fine sediment supply are relatively important compared to river catchments with high connectivity.
In the absence of an active gravel supply or other large, mobile sediment, geomorphic features, such as pool-riffle sequences, are created naturally in chalk rivers by woody debris. Woody debris is derived from overhanging trees at the riparian margin, the species of which may include, amongst others, willow, alder and occasionally oak.
All of these defining features come together to produce perhaps the most valuable characteristic of chalk rivers: high ecological diversity, both in-stream and within the riparian zone. Naturally occurring macrophyte species such as common water-starwart and common water-crowfoot grow well in the fast-flowing waters found in chalk rivers. These and other macrophytes provide cover for numerous macroinvertebrates during their larval stage (e.g. dragonflies and damselflies), as well as providing food for beetles and watervoles, and acting as silt-sediment retention traps. A wide range of fish species are also found in chalk rivers, most notably brown trout and Atlantic salmon but also grayling, brook lamprey and bullhead. Otters may reside within chalk rivers, whilst bird species such as lapwing, snipe and redshank can be found in surrounding floodplain wetlands.
All in all, quite a lot goes into characterising an English chalk stream! However, these traits alone do not explain why these rivers are valuable. In the next post we’ll have a look at why chalk streams are important; ecologically, economically and geomorphologically.